Solution:
Initial velocity \(u = 72\text{ km/h} = 20\text{ m/s}\). Using \(s = ut - \frac{1}{2}at^2\): \(20 = 20(2) - \frac{1}{2}a(2)^2 ⇒ 20 = 40 - 2a ⇒a = 10\text{ ms}^{-2}\).
A car is travelling at \(72\text{ kmh}^{-1}\) and is \(20\text{ m}\) from a barrier when the driver puts on the brakes. The car hits the barrier 2s later. What is the magnitude of the constant deceleration?
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